AWAKE: A Proton Driven Plasma Wakefield Experiment at CERN Edda Gschwendtner, CERN CEA, Saclay, 10 December 2018 Outline • Motivation • Plasma Wakefield Acceleration • AWAKE • Outlook 2 Motivation: Increase Particle Energies • Increasing particle energies probe smaller and smaller scales of matter • 1910: Rutherford: scattering of MeV scale alpha particles revealed structure of atom • 1950ies: scattering of GeV scale electron revealed finite size of proton and neutron • Early 1970ies: scattering of tens of GeV electrons revealed internal structure of proton/neutron, ie quarks. • Increasing energies makes particles of larger and larger mass accessiBle • GeV type masses in 1950ies, 60ies (Antiproton, Omega, hadron resonances… • Up to 10 GeV in 1970ies (J/Psi, Ypsilon…) • Up to ~100 GeV since 1980ies (W, Z, top, Higgs…) • Increasing particle energies probe earlier times in the evolution of the universe. • Temperatures at early universe were at levels of energies that are achieved by particle accelerators today • Understand the origin of the universe • Discoveries went hand in hand with theoretical understanding of underlying laws of nature à Standard Model of particle physics 3 Motivation: High Energy Accelerators • Large list of unsolved problems: • What is dark matter made of? What is the reason for the baryon-asymmetry in the universe? What is the nature of the cosmological constant? … Our Universe Atoms (4%) Dark Matter (26%) Dark Energy (70%) What is this other 96%? • Need particle accelerators with new energy frontier è 30’000 accelerators worldwide! Also application of accelerators outside particle physics in medicine, material science, biology, etc… 4 LHC Large Hadron Collider, LHC, 27 km circumference, 7 TeV 5 Discover New Physics Accelerate particles to Limitations of conventional circular even higher energies accelerators: à Bigger accelerators • For hadron colliders, the limitation is magnet strength. Ambitious plans like the FCC call for 16 T magnets in a 100 km tunnel to reach 100 TeV proton-proton collision energy. Future Circular Collider FCC • For electron-positron colliders: Circular machines are limited by synchrotron radiation in the case of positron colliders. These machines are unfeasible for collision energies beyond ~350 GeV. 6 Discover New Physics Linear colliders are favorable for acceleration of low mass particles to high energies. CLIC, electron-positron collider with 3 TeV energy à 48 km Limitations of linear colliders: • Linear machines accelerate particles in a single pass. The amount of acceleration achieved in a given distance is the accelerating gradient. This number is limited to 100 MV/m for conventional copper cavities. 7 Conventional Acceleration Technology Radiofrequency Cavities LHC Cavity (invention of Gustav Ising 1924 and Rolf Wideroe 1927) Very successfully used in all accelerators (hospitals, scientific labs,…) in the last 100 years. Accelerating fields are limited to <100 MV/m • In metallic structures, a too high field level leads to break down of surfaces, creating electric discharge. • Fields cannot be sustained, structures might be damaged. è several tens of kilometers for future linear colliders 8 Saturation at Energy Frontier for Accelerators Livingston plot è Project size and cost increase with energy 9 Motivation New directions in science are launched By new tools much more often than By new concepts. The effect of a concept-driven revolution is to explain old things in new ways. The effect of a tool-driven revolution is to discover new things that have to Be explained. From Freeman Dyson ‘Imagined Worlds’ 10 Plasma Wakefield Acceleration èAcceleration technology, which obtains ~1000 factor stronger acceleration than conventional technology. 11 Conventional vs. Plasma R&D on feasibility ongoing Technical Design Report prepared Conceptual Design Report prepared 12 Outline • Motivation • Plasma Wakefield Acceleration • AWAKE • Outlook 13 Seminal Paper 1979, T. Tajima, J. Dawson Use a plasma to convert the transverse space charge force of a beam driver into a longitudinal electrical field in the plasma 14 Plasma Wakefield What is a plasma? Plasma is already ionized or “broken-down” and can sustain electric fields e up to three orders of magnitude higher gradients à order of 100 GV/m. e Rb+ + e Rb+ Rb Rb+ Rb+ e Quasi-neutrality: the overall charge of a plasma is about zero. e e e e e Collective effects: Charged particles must be close enough together e Rb+ Rb+ Rb+ Rb+ Rb+ that each particle influences many nearby charged particles. Example: Single ionized rubidium plasma Electrostatic interactions dominate over collisions or ordinary gas kinetics. What is a plasma wakefield? e e e e e e e e e e e e Rb+ e e e e e e + e Rbe + e e Rb e e eRb+ e + Fields created by collective motion of plasma particles are called e e Rb e e e e plasma wakefields. Rb+ Rb+ Rb+ + +e e e Rb e Rb e e e e e e e e e e 15 Plasma Baseline Parameters • A plasma of density npe is characterized by the plasma frequency 2 npe e c ωpe ωpe = è … unit of plasma [m] kpe = √ me ε0 ωpe c 14 -3 12 c -1 Example: npe = 7x10 cm (AWAKE) è ωpe = 1.25x10 rad/s è = 0.2mm è kpe = 5 mm ωpe • This translates into a wavelength of the plasma oscillation c 1015 cm-3 λpe = 2π è λ ≈ 1 mm ωpe pe √ npe λpe = 1.2 mm à Produce cavities with mm size! 16 How to Create a Plasma Wakefield? What we want: Our Tool: Charged particle Bunches carry almost Longitudinal electric field to purely transverse electric fields. accelerate charged particles. e - e e acceleration Rb+ + e Rb+ Rb e- Rb+ Rb+ e drive- e e e e e beam e Rb+ Rb+ Rb+ Rb+ Rb+ Single ionized rubidium plasma Using plasma to convert the transverse electric field of the drive bunch into a longitudinal electric field in the plasma. The more energy is available, the longer (distance-wise) these plasma wakefields can be driven. 17 How to Create a Plasma Wakefield? wave accelerated particles (surfers) drive beam (boat) Plasma (lake) OSIRIS/IST, Portugal 18 Principle of Plasma Wakefield Acceleration • Laser drive beam è Ponderomotive force • Charged particle drive Beam è Transverse space charge field • Reverses sign for negatively (blow-out) or positively (suck-in) charged beam • Plasma wave/wake excited by relativistic particle bunch • Plasma e- are expelled by space charge force • Plasma e- rush back on axis • Ultra-relativistic driver – ultra-relativistic wake à no dephasing • Acceleration physics identical for LWFA, PWFA plasma wavelength λpe 19 Where to Place the Witness Beam (Surfer)? Accelerating for e- Decelerating for e- - e Focusing for e- Defocusing for e- 20 Linear Theory (P. Chen, R. Ruth 1986) When drive beam density is smaller than plasma density (nB << np) à linear theory. • Peak accelerating field in plasma resulting from drive beam with Gaussian distribution: 2 è eEz ≈ N/σz B.E. Blue 2003 • Wakefield excited by bunch oscillates sinusoidally with frequency determined by plasma density • Fields excited by electrons and protons/positrons are equal in magnitude But opposite in phase • The accelerating field is maximized for a value of kpe σz ≈ √2 kpe σr ≤ 1 14 -3 -1 Example: npe = 7x10 cm (AWAKE), kpe = 5 mm è drive beam: σz = 300µm, σr = 200µm 21 Linear Theory • Maximum accelerating electric field reached with drive beam of N and σz: MV N/(2 x 1010) Eacc= 110 Drive beam fulfills: kpe σz ≈ √2 e 2 - m (σz / 0.6mm) Examples of accelerating fields for different beam parameters and plasma parameters fields: 10 14 -3 N = 3x10 , σz = 300µm, npe= 7x10 cm è Eacc = 600 MV/m 10 17 -3 N = 3x10 , σz = 20µm, npe= 2x10 cm è Eacc = 15 GV/m 22 Experimental Results SLAC Experiment, I. Blumenfeld et al, Nature 455, p 741 (2007) High-Efficiency acceleration of an electron beam in a plasma • Gaussian electron beam with 42 GeV, 3nC @ 10 Hz, sx = 10µm, wakefield accelerator, M. Litos et al., doi, Nature, 6 Nov 2014, 50 fs 10.1038/nature 13992 • Reached accelerating gradient of 50 GeV/m • 1.7 GeV energy gain in 30 cm of pre-ionized Li vapour plasma • Accelerated electrons from 42 GeV to 85 GeV in 85 cm. • 6 GeV energy in 1.3 m of plasma • Total efficiency is <29.1%> with a maximum of 50%. • Final energy spread of 0.7 % (2% average) • Electric field in plasma wake is loaded by presence of witness bunch • Allows efficient energy extraction from the plasma wake 23 Many, Many Electron and Laser Driven Plasma Wakefield Experiments…! Now first Proton Driven Plasma Wakefield Experiment 24 Beam-Driven Wakefield Acceleration: Landscape Drive (D) Facility Where Witness (W) Beam Start End Goal beam Use for future high energy e-/e+ collider. CERN, Geneva, 400 GeV Externally injected electron - Study Self-Modulation Instability (SMI). AWAKE 2016 2020+ Switzerland protons beam (PHIN 15 MeV) - Accelerate externally injected electrons. - Demonstrate scalability of acceleration scheme. 20 GeV - Acceleration of witness bunch with high quality Two-bunch formed with SLAC, Stanford, electrons Sept and efficiency SLAC-FACET mask 2012 USA and 2016 - Acceleration of positrons (e-/e+ and e--e+ bunches) positrons - FACET II preparation, starting 2018 PITZ, DESY, 20 MeV No witness (W) beam, only DESY-Zeuthen Zeuthen, electron 2015 ~2017 - Study Self-Modulation Instability (SMI) D beam from RF-gun. Germany beam X-ray FEL DESY, type D + W in FEL bunch. - Application (mostly) for x-ray FEL DESY-FLASH Hamburg, electron Or independent W-bunch 2016 2020+ - Energy-doubling of Flash-beam energy Forward Germany beam 1 (LWFA). - Upgrade-stage: use 2 GeV FEL D beam GeV BNL, - Study quasi-nonlinear PWFA regime. Brookhaven 60 MeV Several